1. Field of the Invention
The present invention relates to a telephone switching system having a redialing feature, and more particularly, relates to a telephone switching system having software incorporated therein for automatic redialing and repeatedly redialing a stored telephone number at periodic intervals until either a ring signal is obtained from a called line corresponding the stored telephone number or a predetermined number of times of which the stored telephone number has been redialed at the periodic intervals.
2. Related Art
Conventional telephone systems are widely used to perform a memory dial function and a redial function. In the memory dial mode, telephone numbers of called subscribers are registered in a memory such as a random-access-memory (RAM) in terms of abbreviated telephone numbers of one or more digits as disclosed, for example, in U.S. Pat. No. 4,119,810 for Pushbutton Telephone Apparatus With Automatic Dialing And Combinational Sequential Control Signalling Capabilities issued to Marin et al., U.S. Pat. No. 5,182,769 for Telephone Set Having A Volatile Memory issued to Yamaguchi et al., U.S. Pat. No. 5,392,340 for Telephone Exchange Apparatus issued to Otsuka, U.S. Pat. No. 5,491,745 for Method And Apparatus For A Dual Mode Keypad Permitting One-Touch Telephone Number Dialing issued to Roeder, U.S. Pat. No. 5,568,546 for Method And Apparatus For Dynamic Abbreviated Dialing Assignment issued to Marutiak. When an abbreviated number of a designated called subscriber is dialed, the telephone number corresponding to the abbreviated number is read from the volatile memory and is sent to the telephone of the called subscriber.
In the redial mode, a last dialed telephone number is temporarily stored when a connection to a destination telephone system is not successful so that a caller may conveniently redial by simply depressing a redial button without inputting the entire digits of the telephone number of a called subscriber. Contemporary practice of automatic redialing of a stored telephone number is disclosed, for example, U.S. Pat. No. 4,324,954 for Dialing Circuits And Methods For Electronic Telephone Sets issued to Taylor, U.S. Pat. No. 5,243,646 for Fast Redial System For A Telephone issued to McCarthy, and U.S. Pat. No. 5,394,462 for Communication Apparatus Having A Redial Function And A Simplified-Dial-Number Registration Function issued to Maemura. The telephone number stored in the redial memory may also be automatically redialed for a long distance call such as disclosed in U.S. Pat. No. 5,268,959 for Telephone Long Distance Redialing Apparatus And Process issued to Hong.
One known problem with the redial feature of the conventional telephone system is that the telephone system is structured to generally store only the last dialed telephone number and the contents of the redial memory are disturbed by any subsequent telephone dialing. That is, if the caller places a call to another subscriber after an unsuccessful call to a desired subscriber, the caller cannot use a redial function to place a call to the desired subscriber after calling the other subscriber. In this situation, the caller must again input the entire telephone number of the desired subscriber. One conventional redial arrangement that seeks not to disturb the contents of the redial memory is disclosed, for example, in U.S. Pat. No. 4,508,939 for Last Number Redial Device issued to Lawson et al. Other known redial arrangements that seek to overcome the above problem are disclosed, for example, in U.S. Pat. No. 4,602,128 for Automatic Redialing Arrangement For Multi-Line Telephone Station Sets issued to Freundlich, U.S. Pat. No. 4,930,155 for Multiple Redialing System issued to Kurokawa, and U.S. Pat. No. 5,136,637 for Method And Apparatus For Automatic Redialing Of Telephone Numbers issued to Rust et al. In Rust '637 and Kurokawa '155, for example, a directory of telephone numbers to be redialed is retained and is visually is provided for the caller's selection.
Another problem with the redial feature of the conventional telephone system is that a redial capability requires the caller to manually go "on" hook and then "off" hook in order to use the redial function if the stored telephone number automatically redialed is busy and the call cannot be completed. One known solution to this problem is disclosed in U.S. Pat. No. 4,887,294 for Telephone Automatic Redial System issued to Ruey-Guang. In Ruey-Guang '294, for example, digital integrated circuits are installed along with switches including a redial initiation circuit for initiating an automatic redial of a stored telephone number contained in a redial memory in response to a dial tone from a called line, and a redial interrupt circuit for interrupting a redial sequence in response to a busy tone from the called line. Redial interrupt circuit of Ruey-Guang '294 is constructed of a series of CMOS logic gates along with a timer for temporarily interrupting the dialing sequence for predetermined periods of time and a counter for interrupting the redialing sequence after a stored telephone number has been redialed a predetermined number of times. While the redialing sequence of Ruey-Guang '294 can be carried out and repeated automatically when the caller closes automatic redial switch and decoder switch in order to eliminate the need for the caller to manually go "on" hook and "off" hook, the hardware requirement, I have observed, is cumbersome and cost prohibitive. More particularly, the hardware construction of Ruey-Guang '294 is not suitable for modem telephone switching system such as private automatic branch exchange (PABX) system in which a central switching unit controls operation of individualized extension telephone sets connected thereto. Accordingly, I believe that a simple and cost effective redialing method of a telephone switching system in terms of software can be contemplated for effectively enabling the calling subscriber to reliably redial a stored telephone number, and if the called line of the stored telephone number is busy, to automatically redial the same telephone number at fixed intervals at least until a ring tone is obtained from the called line or until after the telephone number has been redialed a predetermined number of times.